The present invention relates to fuel injection apparatus for internal combustion engines, and more specifically to electromechanical fuel injector systems and associated electronic controls.
Internal combustion engines traditionally employ one of several methods for introducing fuel into the combustion chambers of the engine. In one method, a carburator is connected to the air intake of the internal combustion engine. As air is sucked through the intake and thus through the carburator, fuel is sucked from a curburator fuel bowl and becomes entrained in the air. The fuel/air mixture is then guided into the various combustion chambers of the engine by an intake manifold.
In a second method, gaining greater popularity today because of its greater precision, high pressure fluid injector nozzles are used to inject fuel either directly into the combustion chambers of the engine or into associated mixing chambers. In many such fuel injection systems, the fuel injectors are entirely mechanical. Increasingly, however, electromechanical systems are instead being used.
Electromechanical fuel injection systems employ solenoid operated fuel injectors for each providing a precisely metered pulse of fuel to a corresponding combustion chamber or mixing chamber at the appropriate time during the cycle of the internal combustion engine. Often, sophisticated electronic control systems are used for controlling the timing, wave shape, and duration of the electrical pulse signals which actuate the solenoids of the fuel injectors. The control systems may be responsive to the temperature of the engine, the throttle position, and other variables as well.
Electronic control systems used in electromechanical fuel injection systems require a reliable and stable source of electric power. In vehicles, the source of electric power comprises of a chemical battery, usually providing a nominal 12 volt DC signal. The battery voltage can vary substantially during normal operation of the vehicle, however. When the engine is operating at normal cruise speeds, the battery voltage may be as high as 18 volts or more due to continuous charging of the battery by the alternator associated with the engine. The battery voltage will normally remain above 8 or 9 volts, but may drop as low as 5 volts or less during cold cranking of the engine.
In the past, the problem of battery voltage variation has been handled by designing the fuel injector solenoids so that they will actuate even under worst-case supply voltage conditions (i.e. during cold cranking of the engine). Designs of this nature do not provide a completely satisfactory solution to the problem because the time necessary for a solenoid to "pull-in" will still be dependent upon battery voltage. When power is applied to a solenoid coil, current flow through the coil will build up gradually due to coil inductance. The solenoid will not actuate, however, until the solenoid current reaches a certain threshold level. The amount of time necessary for the coil current to build up to the pull-in threshold is dependent upon the battery voltage. Thus, it takes longer for a solenoid to turn on when battery voltage is low than it does when battery voltage is high.
Furthermore, the solid state switches used to turn the solenoids on and off dissipate power as a function of the square of the current passing through the switch. Solenoids designed to operate directly from battery voltage tend to require substantial current for normal operation. The switches thus dissipate quite a lot of power.